2.29 STRW - Stress Watts
When a voltage is applied across a transformer winding, a voltage appears
between adjacent turns of the winding.
It is essential that the insulation between adjacent turns can withstand
this voltage, the inter-turn stress.
This is a particular problem on miniature line frequency transformers,
which have a very large number of turns of very fine wire. The enamel
coating is very thin, and is easily scratched.
Also such windings consist of many layers and it is easy for wire to drop
down the edge of the bobbin and lie adjacent to lower layers, exposing the
wire enamel to higher than normal inter turn stress voltages.
The problem of inter-turn stress can also occur on high frequency
transformers. In this case the number of turns may be small, but the
voltage per turn will be very much higher than in a line frequency
Most commonly, the problem of inter-turn stress does not immediately
cause a shorted turn but causes a weak spot, which will cause a
transformer to fail eventually. It is one of the most common reasons for
The Stress Wattage test tests the transformer for breakdown of insulation
by measurement of the energising power of a winding, usually the primary,
with the remaining windings open circuit.
This test applies the voltage, usually twice the normal operating
voltage, and continually measures the power to determine any breakdown by
changes in the measurement.
This test is suitable for sub-miniature line frequency, large line
frequency bobbin wound and some HF transformers. It checks the integrity
of inter turn insulation, the magnetic material and joints.
A constant voltage source is applied to the winding under test.
Both the rms voltage and the power are measured. If necessary, the
voltage can is trimmed to the user specified value, and the measurement
The measurement is repeated many times until the dwell time has elapsed.
The result given is the measured power.
Usually, the energization is at twice the working voltage and frequency,
and has the purpose of trying to create an inter-turn breakdown at a point
of potential weakness in the wire enamel.
This breakdown is detected by an increase in the measured power.